Cargo door — Turkish Airlines 981: "indicated locked" is not LOCKED

cargo_door.cell

// ============================================================================
// CARGO DOOR — Turkish Airlines 981: "indicated locked" is not LOCKED
// ============================================================================
//
// On 3 March 1974, Turkish Airlines Flight 981 — a DC-10 climbing out of
// Paris — lost its aft cargo door at 11,500 feet. The latch cams had
// never gone over-center; a baggage handler had forced the handle and
// the indicator showed LOCKED anyway. The door blew, the cabin floor
// collapsed into the cargo hold, and the control cables running through
// it were severed. All 346 aboard died. Two years earlier the SAME
// failure over Windsor, Ontario had been survivable — and the fix was
// made optional.
//
// The defect: "locked" was an INDICATION derived from handle position,
// not a verified state. Here lock verification is a state you can only
// REACH by actually doing it — the vent-door visual check is the only
// edge into locked_verified, and pressurization is only reachable FROM
// locked_verified. A latched-but-unverified door cannot be pressurized:
// the edge does not exist. Differential pressure is bounded in memory.
// ============================================================================

cell CargoDoor {
    face {
        signal close_door(id: String) -> Map
        signal latch_door(id: String) -> Map
        signal verify_lockpins(id: String) -> Map
        signal pressurize(id: String, psi_diff: Int) -> Map
        signal reopen(id: String) -> Map
        signal door_state(id: String) -> String
    }

    memory {
        // cabin differential pressure, psi — structural limit 9
        psi_diff: Map<String, Int> [persistent, consistent]
        invariant psi_diff >= 0 && psi_diff <= 9
    }

    // ABSENT: latched -> pressurized_flight  (the THY 981 departure:
    //         handle stowed, indicator green, cams NOT over-center)
    // ABSENT: door_closed -> pressurized_flight
    state door {
        initial: door_open
        door_open -> door_closed
        door_closed -> door_open
        door_closed -> latched              // handle stowed
        latched -> door_closed              // unlatch
        latched -> locked_verified          // vent-door VISUAL check passed
        locked_verified -> pressurized_flight
        pressurized_flight -> locked_verified   // descent: diff back to zero
        locked_verified -> latched              // ground: ready to unlatch
    }

    on door_step(id: String, target: String) {
        let r = try { transition(id, target) }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "door", get_status(id))
    }

    on close_door(id: String)       { return door_step(id, "door_closed") }
    on latch_door(id: String)       { return door_step(id, "latched") }
    on verify_lockpins(id: String)  { return door_step(id, "locked_verified") }

    on pressurize(id: String, psi_diff_v: Int) {
        let w = try { psi_diff.set(id, psi_diff_v) }
        if w.error != () {
            return map("ok", false, "reason", "{psi_diff_v} psi exceeds the structural limit")
        }
        let r = try { transition(id, "pressurized_flight") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "door", get_status(id), "diff_psi", psi_diff_v)
    }

    on reopen(id: String) {
        // only a depressurized, unlatch-ready door can open — each step
        // back is its own edge; there is no shortcut from flight to open
        return door_step(id, "door_open")
    }

    on door_state(id: String) { return get_status(id) }
}

cell test DoorTests {
    rules {
        // THE THY 981 departure: door latched, indicator green, but the
        // lock pins never verified. Pressurization does not exist here.
        assert close_door("aft1").ok == true
        assert latch_door("aft1").ok == true
        assert_fails transition("aft1", "pressurized_flight")
        assert pressurize("aft1", 8).ok == false
        assert door_state("aft1") == "latched"

        // the verified path: visual check, then climb and pressurize
        assert verify_lockpins("aft1").ok == true
        assert pressurize("aft1", 8).ok == true
        assert door_state("aft1") == "pressurized_flight"

        // no opening a pressurized door — the shortcut edges are absent
        assert_fails transition("aft1", "door_open")
        assert_fails transition("aft1", "door_closed")
        assert reopen("aft1").ok == false

        // descend, depressurize, walk back down to open — each step real
        assert door_step("aft1", "locked_verified").ok == true
        assert door_step("aft1", "latched").ok == true
        assert door_step("aft1", "door_closed").ok == true
        assert reopen("aft1").ok == true

        // an open or merely-closed door can never be pressurized
        assert_fails transition("aft1", "pressurized_flight")

        // the hull never sees a differential beyond its rating
        assert_fails psi_diff.set("aft1", 15)
        assert_fails psi_diff.set("aft1", -2)
    }
}